Problem: Solve for $z$ : $13 = z - 9$
Solution: Add $9$ to both sides: $ \begin{eqnarray} \\ \hphantom{13 {+ 9}} &\hphantom{=}& \hphantom{z} \\ 13 &=& z - 9 \\ \\ {+9} && {+9} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 13 {+ 9} &=& z \\ \end{eqnarray} $ Simplify. $ z = 22$